Novel fws dc-ac grid connected inverter

ABSTRACT

A new class of DC-AC inverter consists of a buck or two buck converters and two or four low frequency switches, and it achieves ultra-high efficiency, reactive power flow capability, small size and low cost in grid-connected applications.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e) ofProvisional Patent Application No. 62/757,168, filed Nov. 8, 2018, theentire disclosure of which is hereby incorporated by reference.

RELATED ART

The present disclosure relates to a DC-AC power inverter, particularlyfor grid-connected applications. In the past decade, some new powerconversion topologies were proposed for grid-connected applications toachieve higher efficiency, lower cost and smaller footprint. Thefollowing publications are just some of the latest proposed topologiesin this endeavor:

-   1. S. Dutta and K. Chatterjee, “A Buck and Boost Based Grid    Connected PV Inverter Maximizing Power Yield From Two PV Arrays in    Mismatched Environmental Conditions,” in IEEE Transactions on    Industrial Electronics, vol. 65, no. 7, pp. 5561-5571, July 2018.-   2. S. Strache, R. Wunderlich and S. Heinen, “A Comprehensive,    Quantitative Comparison of Inverter Architectures for Various PV    Systems, PV Cells, and Irradiance Profiles,” in IEEE Transactions on    Sustainable Energy, vol. 5, no. 3, pp. 813-822, July 2014.-   3. L. Zhou, F. Gao and T. Xu, “A Family of Neutral-Point-Clamped    Circuits of Single-Phase PV Inverters: Generalized Principle and    Implementation,” in IEEE Transactions on Power Electronics, vol. 32,    no. 6, pp. 4307-4319, June 2017.-   4. J. F. Ardashir, M. Sabahi, S. H. Hosseini, F. Blaabjerg, E.    Babaei and G. B. Gharehpetian, “A Single-Phase Transformerless    Inverter With Charge Pump Circuit Concept for Grid-Tied PV    Applications,” in IEEE Transactions on Industrial Electronics, vol.    64, no. 7, pp. 5403-5415, July 2017.-   5. S. Saridakis, E. Koutroulis and F. Blaabjerg, “Optimization of    SiC-Based H5 and Conergy-NPC Transformerless PV Inverters,” in IEEE    Journal of Emerging and Selected Topics in Power Electronics, vol.    3, no. 2, pp. 555-567, June 2015.-   6. W. Li, Y. Gu, H. Luo, W. Cui, X. He and C. Xia, “Topology Review    and Derivation Methodology of Single-Phase Transformerless    Photovoltaic Inverters for Leakage Current Suppression,” in IEEE    Transactions on Industrial Electronics, vol. 62, no. 7, pp.    4537-4551, July 2015.-   7. Y. Zhou, W. Huang, P. Zhao and J. Zhao, “A Transformerless    Grid-Connected Photovoltaic System Based on the Coupled Inductor    Single-Stage Boost Three-Phase Inverter,” in IEEE Transactions on    Power Electronics, vol. 29, no. 3, pp. 1041-1046, March 2014.-   8. L. Zhang, K. Sun, Y. Xing and M. Xing, “H6 Transformerless    Full-Bridge PV Grid-Tied Inverters,” in IEEE Transactions on Power    Electronics, vol. 29, no. 3, pp. 1229-1238, March 2014.

All of the above-listed publications are hereby incorporated byreference in their respective entireties.

In the meantime, a wide range of power conversion topologies can also befound in a number of patent documents. Below is a exemplary list ofthose U.S. patent documents.

Exemplary U.S. Patent Documents  8,369,113 B2 February 2013 Rodriguez 8,582,331 B2 November 2013 Frisch et al.  8,971,082 B2 March 2015Rodriguez  9,071,141 B2 June 2015 Dong et al.  9,093,897 B1 June 2015Weng et al.  9,148,072 B2 September 2015 Ueki et al.  9,318,974 B2 April2016 Yoscovich et al.  9,413,268 B2 August 2016 Fu et. al  9,584,044 B2February 2017 Zhou et al.  9,627,995 B2 April 2017 Ayai  9,641,098 B2May 2017 Fu et al.  9,692,321 B2 June 2017 Hu et al.  9,806,529 B2October 2017 Fu  9,806,637 B2 October 2017 Fu  9,812,985 B2 November2017 Rodriguez  9,831,794 B2 November 2017 Rodriguez  9,866,147 B2January 2018 Kidera et al.  9,871,436 B1 January 2018 Jiao et al. 9,941,813 B2 April 2018 Yoscovich 10,008,858 B2 June 2018 Garrity10,033,292 B2 July 2018 Rodriguez

All of the above-listed exemplary U.S. patent documents are herebyincorporated by reference in their respective entireties.

Prevailing power conversion topologies, including, e.g., those discussedin, e.g., U.S. Pat. Nos. 8,369,113, 9,941,813, 10,008,858, and10,033,292, which are just some of the examples, in Applicant's view,are still in need of improvement in terms of performance. First, manyprevailing inverter topologies has limitation in regards to reducingswitching losses, thereby causing each to have output inductor L andpassive components that are large in size.

In this aspect, there are a few classes of grid-connected invertertopologies, namely, isolated and none-isolated inverters, mid-frequency(MF) (10 KHz-30 KHz), high frequency (HF) (above 100 KHz) or HF with lowfrequency (LF) switches inverter topologies. FIGS. 2B, 2C, 2F, 2Jillustrate a few MF inverters. The ability of MF inverters to reduceswitching losses is limited due to its MF switching frequencies.However, due to its limitation in reducing switching losses, such a MFinverter's output inductor L and passive components are relatively largein size, which is undesirable. FIGS. 2G, 2I are MF inverters with LFswitches. Those inverter topologies are still limited in terms ofcontrolling its switching losses with its MF switching frequency. As aresult, such a inverter topology still sees its output inductor L andpassive components large in size.

Second, many prevailing inverter topologies has limitation in regards tohaving reactive power flow capability. For those grid-connected invertertopologies, achieving reactive power flow while connecting the grid is,however, desirable. Inverters with the topologies illustrated in FIGS.2A, 2E and 2K, can be operated in HF with LF switches. As a result, forsuch an inverter, its output inductor L can be smaller and higherefficiency can be achieved, thus being relatively okay with theabove-discussed first aspect. However, those topologies do not havereactive power flow capability, therefore limiting them to work at unitypower generation only.

Reactive power flow capability is seen as a very important feature fortoday's inverter technology. FIG. 1E shows three configurations of powerstage cells. Configuration (A) is unidirectional cell, whoseconfiguration is used in the topologies illustrated in FIGS. 2E and 2K.A skilled artisan readily appreciates and recognizes that thisconfiguration can only supports unidirectional current flow, thus beingincapable of having reactive power flow.

Continuing with the discussion on FIG. 1E, configuration (B) of a powerstage cell is a conventional half bridge configuration, which supportsbidirectional current flow that may include reactive power flow. It isworth noting that this configuration is used in almost all otherconventional topologies. Although this bidirectional configuration iseasy to be applied in IGBT type's inverter circuit, the associatedoperation switching frequency is limited. For a modern powersemiconductor device such as a MOSFET, higher switching operationalfrequency is possible. However, due to that high voltage devices haveslow recovery times on a body diode of such a semiconductor device,resulting in an inverter suffering from excess switching losses thatotherwise can be avoided or reduced. As result, a MOSFET operating inconfigure (B) has a potential problem of being conducted inshoot-through, causing the inverter to break down.

Accordingly, there is a need to improve the above-discussed undesirableproblems existed in the conventional power conversion topologies.

BRIEF SUMMARY

In one aspect, the presently disclosed novel conversation topologies, asgenerally illustrated in FIG. 1(A), comprises one or two buckconverter(s). Under the presently disclosed conversion topologies, softswitching can be readily achieved, which is in contrast to H bridge orhalf bridge configurations. During each half cycle of a conductionperiod, there are two switches in the respective conduction path, whichis in contrast to other topologies (where there might have been three ormore switches connected in series). As a result, a better efficiency canbe achieved, thereby improving on the size of the associated outputinductor L and passive components.

In another aspect, under the presently disclosed conversion topologies,each set of output terminals receives low-frequency half wave signals.Thus, undesirable high frequency leakage current is minimized.

In yet another aspect, under the presently disclosed configuration (C)illustrated in FIG. 1E, the involved switches are not connected inseries as of the same phase leg. Thus, the slow body diode does notconduct current, thereby resulting in the power stage to not have ashoot-through issue, while having the capability of reactive power flow.On the other hand, the coupled inductor and the switch T1 causes thepower stage to work equivalent to a half bridge power stage, which isbidirectional. Since there is no two power devices are connected inseries, the body diode reverse recovery issue can be avoided. The switchT1 works in ZVS (zero voltage switching) mode, resulting in switchingloss being small. This then also improves the aspect of the size of theassociated output inductor L and passive components.

The above summary contains simplifications, generalizations andomissions of detail and is not intended as a comprehensive descriptionof the claimed subject matter but, rather, is intended to provide abrief overview of some of the functionality associated therewith. Othersystems, methods, functionality, features and advantages of the claimedsubject matter will be or will become apparent to one with skill in theart upon examination of the following figures and detailed writtendescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The description of the illustrative embodiments can be read inconjunction with the accompanying figures. It will be appreciated thatfor simplicity and clarity of illustration, elements illustrated in thefigures, unless expressly specified, have not necessarily been drawn toscale. Also, any text and/or any numerical data (numbers) appeared onany drawing figures is provided to illustrate an exemplary embodiment orimplementation, and thus is provided for the purpose of illustration andnot for the purpose of limitation. For example, the dimensions of someof the elements may be exaggerated relative to other elements.Embodiments incorporating teachings of the present disclosure are shownand described with respect to the figures presented herein, in which:

FIG. 1A illustrates general form of the presently disclosed DC-ACinverter.

FIGS. 1B and 1C show exemplary choices of the buck converter shown inFIG. 1A.

FIG. 1D illustrates an example of the presently disclosed DC-AC Inverterwith one buck converter and four low frequency switches.

FIG. 1E illustrates comparisons among unidirectional and bidirectionalcell configurations.

FIGS. 2A-2C, 2E-2G and 2I-2K illustrate respective conversion topologiesin the related art.

FIG. 3A illustrates an example of the presently disclosed DC-AC Inverterwith two buck converters and two low frequency switches.

FIG. 3B depicts exemplary timing diagrams in connection with themodulation strategy of the presently disclosed DC-AC converter for FIG.3A.

FIGS. 3C-D illustrate exemplary timing diagrams in connection withmodulating timing for the presently disclosed DC-AC converter for FIG.3A.

FIGS. 4A-4D illustrate four respective operation modes for the presentlydisclosed DC-AC converter for FIG. 3A, including exemplary respectivetiming diagrams associated with four operation modes.

FIG. 4E illustrates an example of the presently disclosed DC-AC Inverterreceiving reactive power flow from a large inductor series or largecapacitor parallel with Vac.

FIGS. 4F and 4G illustrate exemplary respective timing diagrams inconnection with how the presently disclosed DC-AC Inverter handlesreactive power flow from an exemplary inductive load and an exemplarycapacitive load.

DETAILED DESCRIPTION

In the following detailed description of exemplary embodiments of thedisclosure in this section, specific exemplary embodiments in which thedisclosure may be practiced are described in sufficient detail to enablethose skilled in the art to practice the disclosed embodiments. However,it is to be understood that the specific details presented need not beutilized to practice embodiments of the present disclosure. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present disclosure is defined bythe appended claims and equivalents thereof.

References within the specification to “one embodiment,” “anembodiment,” “embodiments”, or “one or more embodiments” are intended toindicate that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present disclosure. The appearance of such phrases invarious places within the specification are not necessarily allreferring to the same embodiment, nor are separate or alternativeembodiments mutually exclusive of other embodiments. Further, variousfeatures are described which may be exhibited by some embodiments andnot by others. Similarly, various requirements are described which maybe requirements for some embodiments but not other embodiments.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. Moreover, the use of the terms first, second, etc. do notdenote any order or importance, but rather the terms first, second, etc.are used to distinguish one element from another.

Those of ordinary skill in the art will appreciate that the componentsand basic configuration depicted in the following figures may vary.Other similar or equivalent components may be used in addition to or inplace of the components depicted. A depicted example is not meant toimply limitations with respect to the presently described one or moreembodiments and/or the general disclosure.

The presently disclosed DC-AC converter is illustrated in general formin FIG. 1A, comprising two buck converters and two low frequencyswitches. Referring to FIG. 1A, the DC source can be photovoltaicarrays, batteries, fuel cells or others. The AC source can be utilitygrid, single-phase electric motors or others. Each front converter canbe any unidirectional converter that can generate half sinusoidalwaveform at the frequency of the connected AC source, including but notlimited to a classic unidirectional buck converter (shown FIG. 1B) and athree-level bidirectional buck converter (shown in FIG. 1C) or any otherkind of buck converter. The two converters can be identical, orcombinations of any buck converters.

FIG. 3A shows a presently disclosed DC-AC converter with classic buckconverter configured with bidirectional cell as an example fordescription. One possible modulation strategy is depicted in FIGS. 3B-D,together with AC voltage and current waveforms. Four possible operationmodes of the presently disclosed inverter in unity power generation withthe described modulation strategy are respectively demonstrated in FIGS.4A-4D.

For reactive power generation mode in connection with the presentlydisclosed topologies, FIGS. 4E-4G provide an exemplary configurationdiagram and examples of reactive power flow associated with theexemplary configuration, respectively.

Mode 1

Referring to FIG. 4A and FIG. 3B, during this mode, T3 operates in PWMin generating an AC waveform using known general configurations thatinvolves one or more suitable inductors and one or more suitablecapacitors. T2 works as a low frequency switch. The converter generateshalf wave AC at point “A”, while T1 and T4 are off. Accordingly, powertransfers from the DC source to inductors L1 to output capacitor C3 andthe AC source. As a result, half wave of the AC sinusoidal is generatedcrosses AC source. As indicated in the timing diagrams, T5, which workssimultaneously with T3 and is coupled to L3, conducts the freewheelingcurrent through forward-biased D1 (otherwise reversed-biased during T3ON periods) during T3 OFF periods.

Mode 2

Referring to FIG. 4B and FIG. 3B, during this mode, which can also beappreciated as a specific sub-mode during Mode 1, T2 and D1 are ON,while T1, T3 and T4 are OFF and T5 is ON. Power transfers from theinductors L1, L3, capacitor C3 to the AC source. D1 keeps thefreewheeling current flow of the inductor L1 due to T5 being on, withinductor L3, which is coupled to T5, engaging T5 to connect thefreewheeling current as well.

Mode 3

Referring to FIG. 4C and FIG. 3D, this mode mirrors Mode 1, except forgenerating the other half wave of the AC sinusoidal. Thus, as a skilledartisan readily appreciate, the operation of Mode 3 corresponds with theoperation of Mode 1, except for that the other low-frequency T1 is ON,T4 operates in PWM, while the corresponding switches of T1 and T4,namely, T2 and T3, are off. Thus, power transfers from the DC source toinductor L2 (which corresponds to L1) to output capacitor C4 (whichcorresponds to C3) and the AC source. As a result, the other half waveof the AC sinusoidal is generated crosses AC source. As indicated in thetiming diagrams, T6 (which corresponds with T5), which workssimultaneously with T4 and is coupled to L4, conducts the freewheelingcurrent through forward-biased D2 (otherwise reversed-biased during T4ON periods) during T4 OFF periods.

Mode 4

Referring to FIG. 4D and FIG. 3D, this mode mirrors Mode 2, except forgenerating the other half wave of the AC sinusoidal. Thus, Mode 4 canalso be appreciated as a specific sub-mode during Mode 3. Accordingly,during this mode, T1 and D2 are ON, while T2, T3 and T4 are OFF and T6is ON. Power transfers from the inductors L2, L4, capacitor C4 to the ACsource. D2 keeps the freewheeling current flow of the inductor L2 due toT6 being on, with inductor L4, which is coupled to T6, engaging T6 toconnect the freewheeling current as well.

Reactive Power Flow Mode

Referring to FIGS. 4E, 4F and 4G, during this mode, load (AC source) isreactive, with either inductive load (lagging) or capacitive load(leading). As indicated in FIGS. 4F and 4G, the output current is out ofphase with the output voltage, the coupled inductor L1 and L3, L2 and L4engage the freewheeling current flow during the intervals when T5 and T4are on. And with the diodes D1 and D2 of the combination conducting thecurrent, the bidirectional current flow is established. See Iac1 andIac2 in FIGS. 4F and 4G. Those currents travel in two directions,showing the bidirectional current flow.

In summary, during the positive sinusoidal cycle (v_(ac)>0), T1 remainsoff and T2 remains on. T3, D1 and T5 turn on and off in a complementaryway to generate required current i_(ac1), whereas T4 and D2 remain off.For the negative sinusoidal cycle (v_(ac)<0), T1 remains on and T2remains off. T4, D2 and T6 turn on and off in a complementary way togenerate required current i_(ac2), whereas T3 and D1 remain off.

While the disclosure has been described with reference to one or moreexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular system,device or component thereof to the teachings of the disclosure withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the disclosure not be limited to the particular embodimentsdisclosed for carrying out this disclosure.

What is claimed is:
 1. An DC-AC inverter apparatus comprising: a set ofone or more buck converters each having a bidirectional cell, saidbidirectional cell comprising two switches not connected in serials,with the second switch conducting freewheeling current while the firstswitch being turned off; and a set of two low-frequency switches coupledto two respective terminals of an AC source, each low-frequency switchgenerating a half wave of an AC sinusoidal.